This invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device and method of driving the display which provides a uniform display with improved contrast.
A known method of driving a liquid crystal display device is the voltage averaging method shown in FIGS. 17, 18(a)-18(c) and 19(a). A matrix display liquid crystal cell, shown in FIG. 17, includes a liquid crystal panel 1 having a layer of liquid crystal material disposed between an upper substrate 2 and a lower substrate 3. A plurality of parallel spaced apart scanning electrodes Y1 to Y6 are disposed on the interior surface of substrate 2 in a lateral direction, and a plurality of parallel spaced apart signal electrodes X1 to X6 are disposed on the interior surface of substrate 3. The intersections of scanning electrodes Y1 to Y6 and signal electrodes X1 to X6 form display elements which may be lit, as depicted with diagonal lines in FIG. 17, or unlit, which are shown with unshaded lines. A liquid crystal display generally has more display elements than the 6.times.6 matrix shown for explanatory purposes in FIG. 17.
Selective voltages or non-selective voltages are applied sequentially to scanning electrodes Y1 to Y6. Scanning electrodes which are impressed with selective voltages are known as selected scanning electrodes. The period for which the particular voltage sequence is applied is known as one frame.
As the selective or non-selective voltages are applied in the particular order to scanning electrodes Y1 through Y6, lighting (lit) or non-lighting (non-lit) voltages are simultaneously applied to signal electrodes X1 to X6. A display element becomes lit if the corresponding scanning electrode is selected and a lighting voltage is impressed on the corresponding signal electrode. If a non-lighting voltage is impressed on the signal electrode, the intersection of the signal electrode and the selected scanning electrode is an unlit display element.
FIGS. 18(a)-18(c) and 19(a)-19(c) show the waveform of voltages applied to a pair of display elements D24 and D23 in FIG. 17,respectively. FIGS. 18(a) and 19(a) show the waveform of the signal voltage applied to signal electrode X2. FIG. 18(b) illustrates the waveform of the scanning voltage applied to scanning electrode Y4 and FIG. 19(b) illustrates the waveform of the scanning voltage applied to scanning electrode Y3. FIG. 18(c) depicts the waveform of the resulting voltage applied to display element D24 (a lit state) at the intersection of the signal electrode X2 and the scanning electrode Y4, and FIG. 19(c) depicts the waveform of the resulting voltage applied to the display element D23 (an unlit state) at the intersection of the signal electrode X2 and the scanning electrode Y3.
In FIGS. 18(a)-18(c) and 19(a)-19(c), F1 and F2 represent frame periods.
In frame period F1:
selective voltage=V0, non-selective voltage=V4 PA0 lighting voltage=V5, non-lighting voltage=V3 PA0 selective voltage=V5, non-selective voltage=V1 PA0 lighting voltage=V0, non-lighting voltage=V2.
In frame period F2:
Further, the following relationships are established: EQU V0-V1=V1-V2=V EQU V3-V4=V4-V5=V EQU V0-V5=n.multidot.V ,
when n is a constant. Alternating current is used in the driving process so that the voltages vary in polarity from period F1 to F2. The time required to invert the polarity is known as polarity inverting time.
As seen from a comparison of FIGS. 18(a)-18(c) and 19(a)-19(c), a display element with a corresponding selected scanning electrode is either lit or unlit depending on whether the voltage applied to the corresponding signal electrode is a lighting (selecting) voltage or a non-lighting (non-selecting) voltage. This driving method is known as the voltage averaging method.
The voltage averaging method is less than completely satisfactory because clear-cut rectangular waveforms are not in fact applied to the display dots elements for several reasons. First, the display element has an electrical capacitance determined by its area, the thickness of the liquid crystal layer and the dielectric constant of the liquid crystal material. Second, both the scanning and signal electrodes are made of transparent conductive films with a typical sheet resistance of several tens of ohms, which implies that the electrodes have a constant electric resistance.
Accordingly, while the voltages generated by the driving circuit may have the clear-cut rectangular waveforms of FIGS. 18(a)-18(c) and 19(a)-19(c), the waveforms become unevenly distorted by the time the voltages are actually applied to the display elements. Thus, there may be an undesired difference between adjoining display elements in the effective waveform of voltages applied thereto, which in turn leads to the problem of uneven contrast.
Another driving method, known as a line inversion driving method, has been proposed to overcome the uneven contrast associated with the voltage averaging method. Disclosed in Japanese Patent Laid-Open Publication Nos. 62-31825, 60-19195 and 60-19196, the line inversion driving method involves inverting the polarity of the voltage applied to the liquid crystal panel multiple times during one frame.
FIGS. 20(a)-20(cand 21(a)-21(c) are waveforms utilized in the line inversion driving method. FIG. 20(a) is the waveform of signal voltage applied to signal electrode X2 of FIG. 17 and FIG. 20(b) is the waveform of scanning voltage applied to scanning electrode Y2. The difference between these two waveforms applied to display element D22 formed by the intersection of signal electrode X2 and scanning electrode Y2 is shown in FIG. 20(c). Similarly, FIGS. 21(a) to 21(c) illustrate the waveform of signal voltage applied to signal electrode X2, the waveform of scanning voltage applied to scanning electrode Y3, and the difference between these two waveforms supplied to display element D23.
As is the case in the voltage averaging method, the line inversion driving method is also less than completely satisfactory. This is due to the fact that the density or contrast of a display element on the scanning electrode to which the selective voltage is applied immediately after inverting the polarity of the voltage applied differs from that of the display elements along other scanning electrodes. For this reason, the linear contrast is uneven. When the line inversion drive method is utilized the position of the scanning electrode undergoing polarity inversion varies with time and a stream of uneven linear contrast appears. This phenomenon in turn causes a considerable decline in the quality of the display of the liquid crystal display device.
Two causes have been determined to explain the uneven linear contrast associated with these prior art liquid crystal driving methods. These causes are as follows, referring to the display mode of FIG. 17 and the waveform of FIG. 21(c) as an example. For explanatory convenience, scanning electrodes Y1 to Y6 are arranged such that after the selection sequence from first scanning electrode Y1 to sixth scanning electrode Y6 is complete, the sequence returns to and repeats scanning from electrode Y1. Also for the example, a polarity inversion based on the line inversion driving method occurs between scanning electrodes Y3 and Y4, although in actuality there may be any number and location of polarity inversions effected.
Liquid crystal display panel 1 provides a so-called positive display wherein the contrast increases as an effective voltage applied to the display element rises. Assuming that V is the absolute value of the difference between the non-selecting voltage and the lighting/non-lighting voltage and n.multidot.V is the absolute value of the difference between the selecting voltage and the lighting voltage, where n is a constant typically having a value between 3 and 50.
The voltage waveform actually applied to display element D23 is illustrated in FIG. 22, drawn with a solid line 23. Waveform 23 is formed by a combination of voltage applied to signal electrode X2 and scanning electrode Y3 on the basis of signal electrode X3 in the display element matrix of FIG. 17. The voltage waveform indicated by a broken line 23a represents the voltage applied to scanning electrode Y2 based on signal electrode X2. As can be seen by comparing the waveform of FIG. 21(c) and waveform 23 drawn with the solid line in FIG. 22, the waveform of voltage actually applied to display element D23 is larger than the voltage applied to signal electrode X2 and scanning electrode Y3.
The reasons for this increase are as follows. Signal voltage waveform 23a indicated by the broken line in FIG. 22 is applied to display element D22. Hence, when the selection shifts from scanning electrode Y2 to electrode Y3, an electric charge amounting to Q.sub.1 is discharged by the capacitor created by display element D22. Q.sub.1 is waveform 23a indicated by the broken line in FIG. 22 and is expressed as follows: EQU Q.sub.1 =nVC-(-VC)=(n+1) VC,
where C is the capacitance of the capacitor. The electric charge quantity Q.sub.2 absorbed by display element 23 is expressed as follows: EQU Q.sub.2 =(n-2) VC-VC=(n-3) VC
Hence, the difference .DELTA.Q between Q.sub.1 and Q.sub.2 is given by:
.DELTA.Q=4VC
As shown in FIG. 17 display elements D22 and D23 are next to each other and form electrically-connected capacitors with a low-valued resistance of the shorter signal electrode, which in this case is X3 (generally, 1 mm or less). Therefore, an electric charge, expressed as Q.sub.1 -.DELTA.Q=(n-3) VC, immediately flows from display element D22 to display element D23, resulting in almost no voltage drop between the two elements.
However, an electric charge of .DELTA.Q flows from scanning electrodes Y2 and Y3 or an end of signal electrode X3 (i.e., from outside into a portion to which the voltage is to be applied). When Q is flowing, the resistance of the scanning electrode and the signal electrode is considerably larger, even though the electrodes depend on the location of the display elements. As a result, the flow of electric charge is hindered. Because the electric charge is not easily discharged, even the voltage on signal electrode X3 is forced to drop when the voltage on scanning electrode Y2 falls from the level of selecting voltage to a non-selecting voltage. Accordingly, the effective voltage between signal electrode X3 and scanning electrode Y3 increases.
In other words, if the difference between charge/discharge quantities before and after the progression is positive, the effective value of the voltage applied to the display element on the next scanning electrode increases. Likewise, if the difference is negative, the effective value decreases. The magnitude of the effective value varies depending on the absolute value of the charge/discharge quantity. Charge/discharge quantities before and after the progression are routinely calculated.
Assume K is the number of all display elements on a particular scanning electrode, N.sub.ON is the number of lit elements, and N.sub.OFF is the number of unlit elements. Thus, display element number K is as follow: EQU K=N.sub.ON +N.sub.OFF
Assume M.sub.ON is the number of lit elements on the next scanning electrode, and M.sub.OFF is the number of unlit elements.
Assume C.sub.ON is the capacitance of the capacitor formed by the lit element and assume C.sub.OFF is the capacitance of the capacitor formed by the unlit element. Then, the relationship therebetween is expressed such as: EQU C.sub.ON &gt;C.sub.OFF
All display elements on the selected scanning electrode are charged with the electric charge quantity Q.sub.1 given by: EQU Q.sub.1 =N.sub.ON n VC.sub.ON +N.sub.OFF (n-2) VC.sub.OFF
The display elements on the next selected scanning electrode are charged with the electric charge quantity Q.sub.2 given by the formula: EQU Q.sub.2 =M.sub.ON n VC.sub.ON +M.sub.OFF (n-2) VC.sub.OFF
Accordingly, the difference between electric charge quantities Q.sub.1 and Q.sub.2 is obtained as follows: ##EQU1## since N.sub.OFF =K-N.sub.ON and M.sub.OFF =K-M.sub.on, therefore EQU .DELTA.Q=(N.sub.ON -M.sub.ON) {n (C.sub.ON -C.sub.OFF)+2 C.sub.OFF } V
Assume I is the difference given by (N.sub.ON -M.sub.ON), and B={n (C.sub.ON -C.sub.OFF)+2 C.sub.OFF } v. The result is: EQU .DELTA.Q=I.multidot.B (b 1)
The polarity of the waveform then inverts simultaneously as the selection shifts, so that the display elements on the selected scanning electrode are charged with the electric charge quantity Q given by: EQU Q.sub.1 =N.sub.ON n VC.sub.ON +N.sub.OFF (n-2) VC.sub.OFF
The next scanning electrode is then selected. With the inverted polarity, the display elements on the selected scanning electrode are charged with the electric charge quantity Q.sub.2 given by: EQU Q.sub.2 =-(M.sub.ON n VC.sub.ON +M.sub.OFF (n-2) VC.sub.OFF)
The difference Q between Q.sub.1 and Q.sub.2 is expressed by: ##EQU2## where N.sub.OFF =K-N.sub.On and M.sub.OFF =K-M.sub.ON, so that ##EQU3## Assume F is the sum of (N.sub.ON +M.sub.ON), and D=2K (n-2) VC.sub.OFF. The result is: EQU -Q=F.multidot.B+D
Therefore, taking the polarity inversion into consideration, the electric charge quantity difference is expressed as: EQU .DELTA.Q=-F.multidot.B-D (2)
It follows from formulae (1) and (2) that the difference I becomes positive when the number of lit elements on the scanning electrode selected is greater than that of lit elements on the subsequently scanned scanning electrode during a selective shift with no polarity inversion, resulting in display elements on the subsequently selected scanning electrode having higher density because of the increased effective voltage. In contrast, if the number of lit elements in the subsequent scanned scanning electrode is larger than that of the scanning electrode prior to the selective shift, the difference I becomes negative, resulting in display elements on the subsequently scanned scanning electrode having a lower density because of the decreased effective voltage. These fluctuations correspond to the absolute value of I.
During a selective shift with polarity inversion, the effective voltage impressed across the display elements on the subsequently scanned scanning electrode invariably diminishes by a constant value. At the same time, the effective voltage decreases by a value corresponding to the difference in F before and after the selective shift.
In other words, the unevenness in contrast corresponds to the difference I between the numbers of lit elements before and after a selective shift with no polarity inversion, whereas if polarity inversion occurs during the selective shift, the unevenness in contrast corresponds both to the difference in the number of lit elements before and after the selective shift as well as to the regular contrasting unevenness.
This first cause of contrast unevenness resulting from a selective shift with polarity inversion is the subtle difference produced during the step of changing the polarity between the signal and scanning voltage waveforms outputted by the actual driving circuit.
The selective voltage is impressed just before inverting the polarity. The magnitude of the voltage of each signal electrode corresponding to a non-selective scanning electrode changes immediately after the inversion has been effected to correspond to the electric charge quantity obtained from formula (2). This change in the magnitude of the voltage is dragged (i.e., lags, does not change instantaneously) on the side of the selective voltage after the polarity inversion.
This phenomenon is shown in FIGS. 23, 24(a)-24(c) and 25(a). FIG. 23 illustrates liquid crystal panel 1 identical with that of FIG. 17 but with a different display contents. FIGS. 24(a)-24(cand 25(a) illustrate voltage waveforms for display elements D33 and D43 shown in FIG. 23, respectively. FIG. 24(a) is the voltage waveform applied to signal electrode X3, FIG. 24(b) is the voltage waveform for scanning electrode Y3, and FIG. 24(c) is the waveform of voltage applied across a display element D33 formed at the intersection of signal electrode X3 and scanning electrode Y3. Similarly, FIG. 25(a) is the voltage waveform applied to signal electrode X4, FIG. 25(b) is the voltage waveform applied to scanning electrode Y3, and FIG. 25(c) shows a voltage waveform applied to an adjacent display element D43 formed at the intersection of signal electrode X4 and scanning electrode Y3.
Characteristic of what occurs when a lighting voltage is applied to a signal electrode, the lighting voltage lags on the side of the selecting voltage just after the polarity inversion, as illustrated in FIG. 24(a). Eventually the effective voltage applied across display element D33 decreases to a degree coinciding with the lag, as shown in FIG. 24(c). When a non-lighting voltage is applied to a signal electrode, the non-lighting voltage also lags on the side of the selecting voltage, as illustrated in FIG. 25(a). Eventually the effective voltage impressed on display element D43 increases to a degree coinciding with the lag, as shown in FIG. 25(c). For this reason, lit element D33 has less display contrast than other lit display elements, whereas unlit element D43 becomes more visible than other unlit display elements. The unevenness on the display is proportional to the electric charge given by formula (2).
The second cause of the contrasting unevenness is the unevenness corresponding to the display contents on the liquid crystal panel.
Uneven contrast in the liquid crystal display can be minimized (such as disclosed in the '750 application) by compensating the scan voltage waveform and/or signal voltage waveform according to the characters or patterns produced on the liquid crystal display. Uneven contrast caused by differences in the shades of gray of the picture elements associated with the first and last scanning electrodes compared to the picture elements associated with the scanning electrodes therebetween can be minimized by applying appropriate compensating voltages to the picture elements. Neither compensation technique, however, addresses unevenness in contrast occurring immediately after the polarity of the voltage applied to the liquid crystal panel has been inverted.
Accordingly, it is desirable to provide a liquid crystal display apparatus which counteracts these causes of uneven contrast in the prior art liquid crystal display devices and, in particular, immediately after the polarity of the voltage applied to the liquid crystal panel has been inverted.